The invention refers to a charge/discharge protection circuit for a rechargeable battery comprising at least one rechargeable cell, with a control logic which opens or closes a load current switch depending on the magnitude of the voltage at the battery terminals, the voltage at the charge/discharge terminals of the protection circuit, and the charge or discharge current. Where that control logic comprises an over-voltage detector which produces a control signal when reaching a voltage limit which depends on the electric strength as determined by the protection circuit. This control signal closes a short-circuit switch which connects the battery terminals via a load current switch in series with a fusible link.
Such a circuit is deemed known from the above referenced Related Patent Application, which is expressly referenced herewith. In particular, this circuit can be disposed between a single lithium-ion cell and the electronics of a mobile phone. It can be produced at favorable cost with a significantly lower electric strength than inherently required in a worst case, because the short-circuit switch closes when reaching an appropriately determined voltage limit. The resulting short circuit current thus safely destroys the fusible link so that the cell is protected from a dangerous overcharge. In contrast to all other circuit elements however, the fusible link cannot be integrated in an IC or only at high cost, because it has to respond very reliably at a tightly toleranced nominal current.
The task of this invention is based on the requirement to create a protection circuit of the above described type which can be integrated on a single chip including the fusible link.
This task is inventively solved, in a first preferred embodiment, by designing a series circuit comprising the load current switch and the fusible link for a fraction 1/n of the nominal current and shunting n of such series circuits in parallel, that the short-circuit switch consists of n switch segments, each of which coupled between the node of the load current switch and the fusible link of the series circuit x (x=1, 2. . . n) and the opposite battery connection, that the over-voltage detector creates in sequential succession for each circuit segment x a separate control signal, and that there is disposed a semiconductor switch in parallel to the charge/discharge terminals of the protection circuit, which the over-voltage detector switches conductively when reaching the predetermined voltage limit.
The central idea of the invention is, therefore, to create the condition where the fuse segments in the case of an over-voltage are reliably destroyed and where the destruction proceeds sequentially from the first to the last fuse segment. This is achieved by dividing the functions of the load current switch, the fusible link, and the short-circuit switch into in parallel arranged T-sections, each of which is designed for only a fraction of the nominal load so that each of the easily integrated fuse segments carry only the respective fraction of the nominal current. It is important in this case that the entire protection circuit or its control logic will not be destroyed before through that unduly high over-voltage, in which case the sequential blowing of the fuse segments would no longer be guaranteed. This is taken care of by the semiconductor switch which immediately short-circuits a recognized over-voltage.
Appropriately, the over-voltage detector receives as input voltage the potential at the charge/discharge terminals. In general, the over-voltage detector could equally well receive as input voltage the voltage from the parallel circuit of the, in the event of an over-voltage, now opened load current switch.
Appropriately, at least those circuits components of the control logic generating the control signals for the switch segments receive their supply voltage from an auxiliary voltage source, in particular from a charged buffer capacitor. This safeguards the function of the protection circuit when the outer over-voltage is short-circuited by the semiconductor switch or if it is so high that it has lead to a destruction of other intended parts of the control logic.
The over-voltage detector may in particular comprise a bistable flip-flop circuit, which switches to its second stable state when the predetermined voltage limit is reached, so that the sequential closing of the switch segments of the short-circuit switch is initiated when the predetermined voltage limit is reached during even a brief interval.
The over-voltage detector may comprise a clock generator followed by a shift register of at least n levels, which, when reaching the predetermined voltage limit, delivers sequentially the control signals for the switch segments of the short-circuit switch.
In a second preferred embodiment of this method of producing the control signals, it is possible that each switch segment of the short-circuit switch has associated with it a voltage detector, which delivers a signal at its output when the voltage at this switch segment disappears as a result of the melting of the associated fuse segment and that the outputs of the voltage detectors are coupled to a logic circuit which comprises a clock generator and which, when exceeding the voltage limit, produces the control signal for the next in line switch segment only when the voltage detector of the preceding switch segment delivers an output signal. The logic circuit can be realized, specifically, with an appropriate number of sequentially connected and with each other coupled flip-flops, as is well understood by those skilled in the art, or an appropriate number of short shift registers. Utilizing shift registers has the advantage in this instance that the control signals do not follow each other immediately, but have a short delay determined by the frequency of the clock generator and the number of shift registers, thus allowing the current path created by the prior melting of the fuse segment to cool down.
In a third preferred embodiment (not shown), the semiconductor switch and/or the switch segment of the short-circuit switch may comprise two in series connected, parallel controlled single switches, in order that this component may also be realized in a low-cost technology offering only lower electric strength.
All circuit components, with the exception of capacitors, are preferably integrated on a chip.